Abstract: Extended Huckel (EHMO) calculations on the molecule H2CSiH2 (silaethylene) and H2SiSiH2 (disilaethylene) have been performed and the results subjected to a Mulliken population analysis to elucidate the factors responsible for the instability of such molecules. These calculations indicate that the C=Si [pi]-bond is exceedingly polar, and that energy mismatching of carbon and silicon p-orbitals is in large part reponsible for the weakness of the [pi]-bond. The relatively high overlap population of the Si=Si [pi]-bond suggests that compounds containing such bonds might be amenable to isolation. These conclusions were reinforced by calculating barriers to rotation about the [pi]-bond via EHMO and CNDO methods; the barrier increases in the order C=Si2H4 and Si2H4 in which the triplet state of the 90[deg]-twisted molecule has lowest energy, the singlet state of twisted H2CSiH2 is lowest and corresponds to the configuration, H2C----Si+H2. Although Si d-orbitals strengthen [pi]-bonds by the formation of p-d hybrids, inclusion of d-orbitals in the basis set decreases the rotational barrier by providing greatly increased bonding capabilities in the excited states.